For a conventional semiconductor device, such as DRAM (Dynamic Random Access Memory) of up to 1M bits, a package structure which comprises a lead frame having a plurality of inner leads, a semiconductor chip having contact pads thereon mounted on the inner leads, wherein contact pads are electrically connected to the corresponding inner leads by bonding wires, has been used. Nowadays, in accordance with a development of a semiconductor device of more expanded memory capacity, other package structures such as COL (Chip On Lead) for up to 4M bits and LOC (Lead On Chip) for up to 16M bits have been used.
A conventional lead frame used for a semiconductor device of LOC package structure comprises a plurality of leads each comprising an inner lead and an outer lead, and an insulating film stuck to the inner leads through which the inner leads are to be mounted on a semiconductor chip. The inner leads and the outer leads are fabricated by etching or pressing. The insulating film may be a resin film, such as polyimide film, on both surfaces of which an adhesive layer of thermoplastic resin or thermosetting resin is formed. The insulating film is punched in a predetermined shape by using a punch and a die, then stuck to the inner leads of the lead frame by heat and pressure.
For assembling a semiconductor device, a semiconductor chip is properly positioned against the lead frame thus formed, then attached to the insulating film by heat and pressure. Next, the inner leads and their corresponding contact pads are electrically connected by bonding wires using a bonding tool, respectively. Finally, the inner leads and the semiconductor chip are covered by a molding resin to expose the outer leads of the lead frame from the molding resin.
According to the conventional lead frame and the conventional semiconductor device, however, there is a disadvantage in that it costs high. For one thing, a variety of punches, dies and sticking machine must be prepared and used for obtaining a variety of lead frames. For another, the insulating film itself is expensive and remaining portion thereof created by punching a particular shape of insulating film is useless and must be wasted.
On the other hand, in stead of using an insulating film, a lead frame having inner leads to which only a varnish-like adhesive is applied to form an adhesive layer thereon, have been proposed. The varnish-like adhesive is made of thermoplastic resin or thermosetting resin dissolved in a solvent, and a predetermined quantity of the varnish-like adhesive is applied to each inner lead with a dispenser or by using screen printing technique. After the application of the varnish-like adhesive finishes, the solvent therein is evaporated at a predetermined temperature for a predetermined duration.
According to the second conventional lead frame, there are advantages set out below:
(a) a minimum and necessary quantity of varnish-like adhesive is able to be applied to only a necessary region of each lead. Therefore, not only an expensive materials and tools, such as polyimide film, specific punches and dies, are not necessary, but also residues of such film are no longer created. As a result, the cost of the lead frame becomes low. PA1 (b) a material such as polyimide film is eliminated, and the total amount of high molecule materials to be contained in a semiconductor package is reduced. This makes the package reduce a quantity of moisture which may be absorbed, and makes it less likely to create a crack in the package due to solder reflow. PA1 a plurality of first leads having a region on which a predetermined quantity of adhesive is applied successively; and PA1 a second lead having a region on which an unstable quantity of adhesive is received. PA1 a plurality of first inner leads having a region on which a predetermined quantity of adhesive is applied successively; and PA1 a second lead having a region which is wider than that of the first inner leads so that an unstable quantity of adhesive is received thereon. PA1 a lead frame comprising a plurality of leads, each lead having a region on which a predetermined quantity of adhesive is applied successively, and means for receiving an unstable amount of adhesive; PA1 a semiconductor chip having a plurality of contact pads, each lead of the lead frame being attached to the semiconductor chip with the adhesive, the plurality of contact pads being electrically connected to corresponding leads, respectively; and PA1 a sealing resin for covering the regions of the leads, the receiving means, and the semiconductor chip. PA1 receiving an initial dispense of the adhesive by the first receiving means; PA1 applying a predetermined quantity of the adhesive on the region of each lead, successively; and PA1 receiving the last dispense of the adhesive by the second receiving means.
According to the second conventional lead frame, however, there is disadvantage in that the adhesive layers formed on the leads become nonuniform, which may cause a defect in mounting a semiconductor chip and wire bonding, and make it difficult to obtain a thinner semiconductor package.
For example, when an application of varnish-like adhesive to one of leads begins, the quantity of adhesive dispensed from a needle of the dispenser tends to be unstable, typically more than a predetermined normal quantity to be applied thereto. This is because an undesirable drop of adhesive may be created before the application begins, and a time lag of the dispensation may exist due to a delayed response to the air pressure added to the dispenser. A similar phenomenon may be observed for an adhesive layer formed on the lead where the application ends, because the quantity of the varnish-like adhesive may also become unstable, typically more than the predetermined normal quantity, due to an inertial effect of the adhesive. Such applications of unstable quantities of adhesives to the leads make the adhesive layers on the leads nonuniform.